This invention relates to ultrasonic instrument inserts, more particularly, to magnetostrictive inserts for ultrasonic dental instruments, and especially to such inserts that are power-matched to the power range of a power-sensitive tip.
Ultrasonic instruments such as dental scalers are well known. The ultrasonic scalers include a generator that electrically induces the scaling tip to vibrate at a very high rate. An alternating current is passed through an induction coil in a handpiece that induces vibration of the magnetostrictive element of an insert in the handpiece. The vibration is transmitted to the tip by a velocity transducer on the end of the magnetostrictive element. The vibration may cause the tip to move elliptically, in a curved linear fashion, or in a “figure eight” pattern.
Some tips operate at a lower frequency, while other tips operate at a higher one. The tip, for example, may vibrate 25,000 times per second (25 KHz). If a tip is connected to an insert and used with a generator and handpiece that operate at the improper frequency or power output, the tip may not operate correctly, or can break during operation.
Many current inserts are employed with new special-use tips that are relatively thin compared to conventional thicker tips. The thickness of the tip and the ultrasonic power setting used will affect the vibration of the tip. Generally, a lower power should be used to vibrate thinner tips, and a higher power used with thicker tips. Unless modulated in some manner, usually by manually adjusting a power setting control such as a knob connected to a potentiometer, the ultrasonic power required to vibrate the thicker tips can overstress the thinner tips, creating metal fatigue and causing tip breakage, sometimes within a matter of seconds. Other special-use tips may vibrate excessively at the typical maximum ultrasonic power levels, making them difficult for the dental practitioner to use, and/or causing patient discomfort. Manufacturers typically recommend a power output setting at less than full power, e.g. 50% or 25% of maximum, for these relatively thinner or special-use tips. Ultrasonic tips labeled with manufacturer-recommend reduced-power settings or which are otherwise commonly used at reduced power settings are collectively referred to herein as “power-sensitive tips.” These power-sensitive tips are used in inserts that generally require a minimum or threshold power input before vibration will begin. Above the threshold, the magnitude of the vibration generally increases proportionally with the power supplied. At some point, however, the power input will become excessive in that a sharp increase from the normally linear stress-strain curve for the material of the tip will occur, resulting in accelerated failure of the tip, and/or in that the tip becomes difficult to use or begins to cause patient discomfort. The range of power input from the threshold to the point just before the power input becomes excessive is referred to herein as the “power range” of a particular tip.
Manufacturers and dental practitioners skilled in the art can readily determine the power sensitivity of tips, and the approximate power range in which a specific tip is operable by determining the stress-strain curve and/or by trial and error. Generally, a power sensitive tip is attached to an ultrasonic handpiece comprising a conventional magnetostrictive element connected to a power supply, and operated at increasing levels of power output, from initial tip vibration until metal fatigue, tip breakage, operational difficulty, patient discomfort, or the like occurs. Using this technique, manufactures and users are readily able to determine the minimum and maximum power setting, for which a tip is operable, referred to herein as the “power range.” To avoid overpowering power-sensitive tips, their manufacturers may recommend to the operators to use only a lower power setting on the ultrasonic generator unit, such as half power or less. Manufacturers of ultrasonic power supplies generally provide operating instructions suggesting a power setting limit for a specific tip based on the power conversion efficiency of a conventional magnetostrictive element. These guidelines are intended to prevent overpowering of the tip and resultant metal fatigue, tip breakage, operational difficulty and patient discomfort. This has been only partially effective due to operator error, for example the operator may not remember to reduce the power setting before use, as well as complications due to the variations in the numerous makes and models of many manufacturers and the differences in the circuitry of their ultrasonic dental generators.
The stack material normally used in ultrasonic dental scalers is a form of nickel in the shape of strips or leaves. The nickel can be very pure such as Nickel 33 or Nickel 200, or have other metals or materials alloyed with it to enhance its ultrasonic performance, such as, for example, PERMANICKEL alloy. The surfaces of the nickel strips are typically oxidized to establish a coating, which enhances the ultrasonic efficiency of the strips. The coating also serves to avoid significant variations in efficiency over the useful life of the stack since the surfaces would oxidize at the aqueous conditions of use and autoclave sterilization. As used herein, “nickel” refers to nickel and magnetostrictive nickel alloys, optionally with oxidized surfaces, unless the context indicates otherwise. The stacks made from these optimum materials are usually highly efficient, typically capable of converting 50% or more of the electrical power input to mechanical oscillatory power at the peak resonant frequency, referred to herein as “high-efficiency stacks.” The characteristics of “power-sensitive tips” and “power range” as defined above are determined herein as commonly understood in the art, i.e. when the tips are used in an insert with high-efficiency stacks, unless indicated otherwise by context.
An ultrasonic dental instrument, which operates in a manner to achieve peak efficiency and at maximum energy conversion, is known, for example, from U.S. Pat. No. 3,636,947 to Balamuth. Generally, to achieve vibration of the tip, the magnetostrictive element and surrounding components are structured to allow the ultrasonic unit to operate with high efficiency and minimum disruption of power conversion. The device is generally unconcerned with protection of power-sensitive tips, as the efficiency of the magnetostrictive element is optimized independent of the tip used. Generally, there is no method, other than reducing power output of the power supply, e.g. with a manually adjusted potentiometer, to prevent metal fatigue, tip breakage, operational difficulty, patient discomfort, or the like occurring due to tip power-sensitivity.
An apparatus for adjusting the power to an ultrasonic dental insert by use of a control switch is disclosed in U.S. Pat. No. 3,691,437 to Andersson. The power output of the tool is controlled by a manually operated control switch, which is installed on the outer surface of the dental tool. The control switch causes a short circuit when activated, is similar to reducing the power output at the power supply, and is under the control of the operator. The reduced magnitude of oscillation of the tip is unrelated to tip sensitivity and, therefore does not provide a method of automatically preventing overpowering power-sensitive tips and the concomitant metal fatigue, tip breakage, operational difficulty, or patient discomfort.